Why Are There No Holes Around Trees?

0:00 Trees are the heaviest and largest living things on Earth,

0:04 with the most massive tree weighing almost

0:07 2,000 tons– as much as ten blue whales.

0:10 But instead of floating weightlessly in the ocean

0:13 it reaches 25 stories into the sky, held in place by surprisingly shallow roots.

0:19 You’d think trees grow from the ground because, well,

0:22 they are made of stuff and there is stuff in the ground–

0:25 but if something so massive and huge ate something down below,

0:28 it would have to leave holes.

0:30 Instead trees are growing by literally eating thin air!

0:35 But this is only half of the story because down in the ground,

0:38 roots are mining rocks in ways weirder than you can imagine.

0:42 How does this work?

0:44 How to Eat Air to Grow Huge Carbon

0:47 is the most valuable material for living things.

0:50 A chemical multi tool you can make almost everything from– and a good

0:54 amount of it just floats around in the air and the oceans.

0:58 What makes plants so incredibly successful is that over a billion years

1:02 ago their ancestors became better than

1:04 any other living thing at harvesting carbon.

1:07 They used it to grow and grow and grow.

1:10 Today plants make up 80% of the biomass on earth and are the basis

1:15 for all complex life– all animals eat either plants or animals that eat plants,

1:20 to get the carbon they need.

1:22 Trees are an especially ingenious way plants

1:25 found to harvest massive amounts of carbon.

1:29 Trees are big and heavy and so they need a lot of material.

1:32 But the atmosphere is only about 0.04% CO₂.

1:37 425 CO₂ molecules per million molecules in the air.

1:41 To get a single tonne of carbon a tree has to process 6000 tons,

1:46 or 5 million cubic meters of air!

1:49 This is a lot.

1:51 So trees developed sophisticated

1:54 biological industrial megalopolises: Their crowns.

1:58 A huge industrial park network, made from dozens of branches,

2:02 subbranches and hundreds of thousands of twigs that can sense the sun

2:07 and shape the tree to grow towards it in slow motion.

2:10 They are carrying up to a million leaves,

2:13 the industrial parks where a tree eats and builds,

2:16 consuming extreme amounts of resources from the air and fed by the roots

2:20 down below while vomiting waste and changing the climate around them.

2:25 Let’s zoom into a single leaf.

2:27 It is made from hundreds of millions of factory cells and optimized to have

2:32 as much surface area and be as thin as possible to harvest sunlight.

2:36 While your skin is hundreds of cells thick,

2:39 a leaf can be just ten cells top to bottom.

2:42 On their top leaves have only a single,

2:44 ultra thin layer of protective transparent “skin” cells

2:47 that let light through and keep water in.

2:51 Below them are layers of factory cells,

2:53 filled to the brink with chloroplasts that do the actual work.

2:57 Beneath them a spongy layer of loose cells enables gases to travel around.

3:02 The whole leaf is traversed by a network of vein-like superhighways,

3:05 that carry sugars back down and bring water and minerals up from the roots.

3:10 At the bottom is another protective layer

3:13 of cells interrupted by hundreds of thousands of stomata–

3:16 tiny mouths opened and closed by two guard cells that look a bit like lips.

3:21 Each day an adult tree pulls up dozens of liters of water

3:24 all the way from its roots in the ground to these veins,

3:27 where about 95% of it is sweated out

3:30 through hundreds of billions of these tiny mouths.

3:33 This cools the leaf factories,

3:34 which need to stay in direct sunlight as long as possible,

3:37 and the air around the tree.

3:39 And it surrounds the tree with an invisible mist.

3:42 The vapor from a forest of billions of trees can seed clouds and create rain.

3:48 Rainforest is literal– without the trees the amazon

3:52 would be a sad dry shrubland or desert.

3:56 The other 5% of the water is used to keep the cells

3:59 alive and to power the factories where the magic happens: Photosynthesis.

4:04 We are not going to explain the details here, but in a nutshell,

4:07 with the energy from the sun,

4:09 water molecules are split into hydrogen and oxygen.

4:12 The oxygen is ejected,

4:14 while the leftover hydrogen and CO2 are forged and reduced

4:17 into glucose– a simple sugar that’s both battery and building block.

4:22 And the source of most carbon in the world for most animals.

4:27 Oxygen is not just garbage to the tree though.

4:30 To actually use the energy stored in the glucose,

4:33 the tree has to burn the sugar,

4:35 just like we humans do, with cellular respiration.

4:38 So all living cells in the tree suck in oxygen– through the tiny leaf mouths,

4:43 cracks in the bark, and even root tips

4:45 tapping into tiny air pockets hidden in the soil.

4:48 This respiration runs nonstop,

4:50 and especially at night when the leaf factories stop production.

4:55 Trees actually reabsorb some of the oxygen they produce and almost all

4:59 of the rest gets used up by microbes and everything else breathing nearby.

5:04 Most of the world's free oxygen doesn't come

5:06 from trees but from algae and cyanobacteria in the oceans.

5:11 But this is only half of the story because the even

5:14 more insane parts of trees are the second invisible crown:

5:18 the underground empire of the roots.

5:21 Most of the water a tree needs comes from rainfall,

5:24 which soaks mainly into the upper layers of soil.

5:27 Annoyingly for trees their crowns are big umbrellas,

5:30 so their roots need to spread out far and wide towards the side.

5:33 About 50% of their roots are packed into the top 25 centimeters of soil.

5:38 They are not a mirror image of the crown,

5:40 but a dense, tangled mat, deeply intermingled with their neighbours’.

5:44 Only if it’s very dry do roots grow straight down to tap hidden water reserves,

5:49 in extreme cases more than 20 stories deep.

5:52 But this is a rare exception.

5:54 Most roots reach down 7m.

5:57 But roots have a far more complex job than just catching water.

6:00 Just like you can’t build a city from only bricks and steel,

6:04 trees also need some rare materials:

6:06 phosphorus to build DNA, nitrogen for proteins and many more.

6:10 And all of these are stealthily buried underground.

6:13 Rocks, dry patches, nutrients,

6:15 and rival roots are all scattered unpredictably and chaotically.

6:19 To navigate this shifting maze,

6:21 roots evolved a specialized sensor at their very tip: the root cap.

6:26 Each cap is filled with gravity-sensing cells,

6:29 in which tiny dense particles sink like pebbles settling in a jar of water.

6:34 So the root always knows which way is down.

6:37 As it pushes forward, specialized cells detect moisture,

6:41 temperature, chemical gradients and the smallest vibrations from water.

6:45 This raw data flows into the root’s command center just behind the tip,

6:49 where cells produce electrical pulses and move transmitter chemicals around.

6:54 Signals from the soil are processed,

6:56 interpreted and turned into decisions about where to grow.

7:00 A single tree has hundreds of thousands of these command

7:03 centers and they seem to share information with each other.

7:06 Once a root has chosen a path, fuzzy little drinking straws called root hairs,

7:11 loaded with enzymes and transport proteins,

7:13 begin soaking up water and dissolved minerals.

7:17 But many essential nutrients are locked away in solid rock.

7:21 So roots evolved to move into the finest cracks.

7:24 Once in, they fill with water and swell like tiny hydraulic jacks,

7:28 creating enough pressure to break even the hardest rock.

7:32 Next they release a mix of acids that seep

7:34 into the fractures and dissolve the bonds that hold nutrients in place.

7:38 Claw-like molecules grab them and pull them in before they can slip away.

7:42 This sophistication really is stunning but it gets even wilder.

7:47 Even with all these tools, to really thrive, the tree needs allies.

7:52 And it found them: fungi.

7:55 The underground networks of fungi can stretch for kilometers.

7:59 They are so small that they can go where roots can’t,

8:01 slipping between grains of soil to reach distant pockets of nutrients.

8:05 But they need food.

8:07 So hundreds of millions of years ago roots and fungi formed a trade alliance.

8:12 The trees provide a cut of the sugars they produce far up

8:16 in the sky and fungi collect and give them nutrients and water in return.

8:21 Some fungi grow directly into the root’s cells,

8:24 building tiny trade posts, where sugars and minerals change hands.

8:28 Others wrap themselves around root tips, weaving between their outer layers,

8:33 insulating delicate tissues and protecting them against microorganisms.

8:38 Today there are thousands of fungal tree ally species,

8:41 each with its own specialties.

8:43 Some only partner with specific tree species,

8:46 while others are happy to work with almost anyone.

8:49 These connections often knit the roots

8:52 of many trees together into vast underground networks.

8:55 Their scale is gigantic.

8:57 In just one cubic meter of healthy forest floor,

9:00 fine tree roots can stretch for several

9:02 kilometers and for every kilometer of root,

9:05 there can be hundreds of kilometers of fungal networks.

9:09 It’s one of the largest and most intricate living structures on Earth,

9:13 and may even connect whole forests.

9:16 We are only beginning to understand how

9:19 complex and intricate the relationships between trees,

9:21 their offspring, relatives and rivals, microbes and fungal networks are.

9:27 But the more we’ve learned over the last few decades,

9:30 the clearer one thing has become: Trees are just so incredibly wild.

9:35 And we have so much more to learn.

9:40 People once believed that all fungi— even

9:43 those allied with trees— were just strange plants.

9:46 But then, some incredible minds discovered that they

9:48 were actually an entirely different life form!

9:51 This breakthrough in understanding was powered

9:53 by a combination of technical knowledge and problem-solving skills.

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